Metal fine particles have been conventionally used for producing an electrical wiring material for electronic parts in the form of a conductive paste for use in, for example, printed wiring, interconnects on semiconductor devices, and contacts on a printed wiring board to electronic parts. Particularly, unlike typical submicron or larger particles, metal fine particles having a particle size of 100 nm or less can be sintered at an extremely low temperature, and therefore the application of such metal fine particles to low-temperature sintering pastes and the like has been contemplated.
Recently, it is particularly focused on a technique that forms an electrical wiring by sintering, at a low temperature, a wiring pattern printed in a metal fine particle-containing ink by an inkjet printer, and this technique is being actively researched and developed. An inkjet printer ink for use in this technique is required to maintain the dispersibility of metal fine particles contained therein for a long period of time, and therefore the particle size needs to be even smaller as compared with the conventional metal fine particles. It is to be noted that in the case of a pigment-based ink now practically used in the inkjet printer, an organic pigment or carbon black contained therein is generally required to be 50 to 200 nm in particle size.
On the other hand, metal fine particles for use in inkjet printer ink are desired to have a particle size of 50 nm or less from the viewpoint of dispersibility. The reason for this is as follow. For example, in the case of an inkjet printer ink containing copper fine particles as metal fine particles, the density of an organic pigment or carbon black is 1.5 to 2.5 g/cm3, whereas the density of metal copper is 8.96 g/cm3 that is about 4 to 6 times that of the organic pigment or carbon black. Therefore, in order to achieve the same degree of precipitation velocity as the organic pigment or carbon black, the copper fine particles are required to have a particle size of about 12.5 to 50 nm, which is determined in consideration of the above-described density difference using the known Stokes equation for calculating the precipitation velocity of a fine particle in a dispersion, assuming a dispersion medium is water and the density of the organic pigment is 1.5 g/cm3.
In the case of a conductive paste for use as an electrical wiring material for electronic parts, it is known that impurities remaining in wiring cause a problem and that halogen is particularly harmful. More specifically, the presence of a large amount of impurity element in wiring promotes corrosion of a wiring metal, which causes migration or movement of a metal element to an insulated area. Consequently, insulation failure is likely to occur. The influence of such migration on recent fine-pitch patterns of wiring for electronic devices is more serious than that of conventional ones. Unlike the sintering process at a high temperature, the sintering process at a low temperature, particularly at 300° C. or less, offers little chance to remove an impurity element by volatilization during sintering. Therefore, it is necessary to avoid contamination of a conductive paste by undesirable impurities as much as possible.
As a method for producing metal fine particles for use in conductive paste, a chemical method has been proposed which produces metal fine particles in a solution phase. In general, the chemical method reduces a metal compound by using a reducing agent in a solution. A polyol process is well known as a method for synthesizing metal fine particles in a concentrated system adjusted to the mass production. As disclosed in, for example, Japanese Patent Application Laid-Open No. 59-173206, a polyol process heats and reduces an oxide of copper such as copper oxide or a salt of copper to copper metal in a polyol. The polyol in this process plays three roles of a solvent, a reducing agent, and a protective agent.
The polyol process makes it possible to obtain metal fine particles having a particle size of the order of submicrons to microns even in a concentrated system. Particularly, by using a metal compound, such as a metal oxide or a metal hydroxide, as a starting material, it is possible to obtain metal fine particles that do not contain undesirable impure elements for industrial application. Further, it is known that metal fine particles having a very small particle size can be obtained by appropriately selecting the kind of polyol used, reaction temperature, and raw materials. However, it is very difficult for the conventional polyol process to synthesize metal fine particles, especially copper fine particles, having both a particle size of 100 nm or less and high dispersibility.
Further, JP-A No. 2003-166006 proposes a method for producing copper fine particles using a polyol process, in which copper compounds having a particle size of less than 200 nm are suspended in a polyol solvent and are then reduced under pressurized hydrogen at a temperature of less than 150° C. However, as described above, this method needs heating under pressurized hydrogen, which not only requires the use of a complicated apparatus but also involves danger. Further, the particle size of obtained copper fine particles is about 50 nm at minimum.
Further, a method for producing copper fine particles having an average particle size of 50 nm or less using a polyol process has been proposed, in which copper oxide or copper hydroxide is used as a starting material and noble metal ions are added as a nucleation agent. For example, JP-A No. 2005-307335 proposes a method for producing copper fine particles, in which an oxide, hydroxide, or salt of copper is heated and reduced in a solution of ethylene glycol, diethylene glycol, or triethylene glycol containing noble metal ions for nucleation, polyvinylpyrrolidone as a dispersant, and an amine-based organic compound as a reduction reaction controller. Further, JP-A No. 2005-330552 proposes a method for producing copper fine particles, which is the same as the method disclosed in JP-A No. 2005-307335 except that palladium ions and polyethyleneimine are respectively added as a nucleation agent and a dispersant to the polyol solution before the heating and reducing step.
These methods make it possible to obtain copper fine particles having an average particle size of 50 nm or less, but the use of palladium ammonium chloride or palladium chloride for nucleation makes it impossible to avoid contamination of the copper fine particles by an undesirable halogen. Further, in these methods, an alkaline inorganic compound can be added if necessary, but this may unfavorably create copper fine particles contaminated by an undesirable alkali metal element. Among various undesirable elements, chlorine ions are particularly strongly adsorbed to generated copper fine particles, and therefore cannot be removed even by washing after synthesis. Further, the polymer coating layer over the copper fine particles becomes a factor that inhibits sintering, and therefore if the polymer coating layer is too thick, the sintered copper fine particles cannot have satisfactory conductivity, on the other hand, if the polymer coating layer is too thin, the copper fine particles are poor in long-term dispersibility in the solution and lose oxidation resistance.
Further, JP-A. Nos. 2005-097677 and 2000-123634 describe that a conductive paste is sintered in a hydrogen atmosphere or vacuum atmosphere to form wiring. However, such firing in a hydrogen atmosphere or vacuum atmosphere requires the use of an apparatus having a complicated structure and creates concerns about safety. Under the circumstances, there is a demand for an ink or a paste which contains copper fine particles which can be sintered under simpler conditions. Further, the above-described methods include sintering at a high temperature of 250° C. or higher, which limits substrate materials that can be used. Therefore, there is a demand for development of a paste that can be sintered at a lower temperature.
Patent Document 1: Japanese Patent Application Laid-Open No. 59-173206
Patent Document 2: JP-A No. 2003-166006
Patent Document 3: JP-A No. 2005-307335
Patent Document 4: JP-A No. 2005-330552
Patent Document 5: JP-A No. 2005-097677
Patent Document 6: JP-A No. 2000-123634